Nowadays, 50˜150 ml small displacement, rear-positioned, manual-clutch horizontal engines are widely used on motorcycles, cub motorcycles and motor tricycles. When the engine starts, the starting gear on the starting shaft rotates the first gear on the main shaft through the idle gear on the countershaft, the first-gear rotates the main shaft, the main shaft rotates the driven gear on the clutch housing, the driven gear then rotates the drive gear on the crankshaft, and thus the engine is started. However, when this series engine shuts off and re-starts, it needs to change to neutral first, that is, to engage the first gear on the main shaft with the idle gear on the countershaft again. Limited by the present structure of the main shaft and countershaft, the user is required to move the motorcycle back and forth to facilitate downshifting, thus it brings great inconvenience for the user in practice, especially for the user of motor tricycle. Because the deadweight of the motor tricycle is heavy, it is effort consuming and very inconvenient for the user to move back and forth.